The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines can operate in a variety of different combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines can be configured to operate in a homogeneous-charge compression-ignition combustion mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. The controlled auto-ignition combustion comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the controlled auto-ignition (HCCI) combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. Controlled auto-ignition combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of an air/fuel stoichiometric point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
In a stratified-charge SI combustion mode, the engine operates at a lean air/fuel ratio with the injected fuel mass stratified in the combustion chamber with rich layers proximal to the spark plug tip and leaner air/fuel ratio areas distal thereto. Fuel injection timing is preferably close in time to spark timing to prevent the air/fuel mixture from homogenizing into a uniformly disbursed mixture. The fuel injection pulse width ends as the spark event begins or substantially prior. Upon ignition, the rich layers burn quick and efficiently. As the combustion process proceeds into the leaner areas, the flame-front cools rapidly decreasing overall combustion temperatures and reducing NOx formation.
Known controlled auto-ignition combustion strategies may include using an exhaust recompression valve strategy. The exhaust recompression valve strategy includes controlling a cylinder charge temperature by trapping hot residual gas from a previous engine cycle by adjusting valve close timing. In the exhaust recompression strategy, the exhaust valve closes before TDC and the intake valve opens after TDC creating a negative valve overlap (NVO) period in which both the exhaust and intake valves are closed, thereby trapping the exhaust gas. The opening timings of the intake and exhaust valves are preferably symmetrical relative to TDC. Both a cylinder charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from a previous cycle can be retained with earlier closing of the exhaust valve leaving less room for incoming fresh airflow, thereby increasing cylinder charge temperature and decreasing cylinder oxygen concentration.
In engine operation, the engine airflow can be controlled by selectively adjusting position of the throttle valve and adjusting opening and closing of intake valves and exhaust valves. On engine systems so equipped, opening and closing of the intake valves and exhaust valves are accomplished using a variable valve actuation system that includes variable cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes which provide two or more valve lift profiles. A switch in the valve lift of the multi-step valve lift mechanism is a discrete change.
When an engine operates in a controlled auto-ignition (HCCI) combustion mode, the engine control comprises lean air/fuel ratio operation with the throttle wide open to minimize engine pumping losses. When the engine operates in the SI combustion mode, the engine control preferably comprises stoichiometric air/fuel ratio operation, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow.
In an engine configured for operating in SI and controlled auto-ignition (HCCI) combustion modes, transitioning between combustion modes can be complex. The engine control module must coordinate actuation of multiple devices to provide a desired air/fuel ratio for the different modes to maintain combustion stability. During a transition between controlled auto-ignition (HCCI) combustion mode and SI combustion mode, switching the engine valve lift occurs nearly instantaneously, whereas adjusting the variable cam phasers and the throttle introduces response times that result in slower dynamics.